The present invention relates to a method of determining the position and/or motion of an object using an array of radiation detectors. The invention will be described below with reference to arrays of pyroelectric detectors but may be equally applicable to some other arrays of radiation detectors.
A pyroelectric sensor is composed of a thin piece of pyroelectric material with electrodes on the top and bottom surfaces. The pyroelectric material has the property that changes in incident (heat) energy are translated to electrical signals that can be taken from the electrodes via a suitable amplifier for signal processing.
One of the most common detectors of human movement is the Passive Infrared (PIR) detector used in intruder detectors and movement triggered automatic lights. Conventional PIR detectors use a small number of pyroelectric sensors in conjunction with an optical arrangement that defines the field of view and provides a modulated signal from a moving human, as described in more detail below. One consequence of this arrangement is that it is not possible to determine the location of the object within the field of view of the detector, and another is that gaps must be provided within the overall field of view for the detection method to operate, resulting in blind spots.
A solution to these shortcomings can be found by replacing the conventional pyroelectric sensor with an array of pyroelectric detectors and a unitary optical system. By tracking the movement of an object between adjacent detectors of the array, the angular position of the object with respect to the detector is known. This detection method is also outlined below. The use of an array also provides continuous coverage throughout the field of view.
The present invention provides a means for enhancing the performance of an array-based detector, primarily by allowing the detection of movement of an object within the field of view of a single detector in an array of detectors.
With conventional PIR detectors, it is normal for the detector to comprise a pyroelectric sensor with 1, 2 or 4 sensitive detectors, an optical device defining the field of view of these detectors, an amplifier and signal processing circuitry.
The optical device is usually an array of lens segments arranged to direct the field of view of the sensor into a number of finger-like detection zones as shown in FIG. 1(a). When there is only a single detector in the pyroelectric sensor, each lens segment projects one detection zone, but when there are two or more pyroelectric detectors, each lens segment will project a detection zone for each detector in the sensor. FIG. 1(a) shows the most common arrangement, where there are two detectors in the sensor 1 and each lens segment A, B, C, D, E projects a pair of detection zones. The gaps in the coverage pattern can be seen between these detection zones.
The pyroelectric detectors are arranged so that one provides a positive signal when the heat from the object is focused upon it, while the other provides a negative signal when the heat is focused upon it. As is shown in FIG. 1(a), each lens segment will project a pair of detection zones, one with a positive sense and the other with a negative sense. The nature of the pyroelectric sensors is such that they detect changes in incident radiation but ignore steady state radiation.
As a person moves across the field of view of the arrangement described in FIG. 1(a), in the direction of arrow X, the radiation (heat) from the person is sensed when it is in one of the detection zones, and is lost when it moves into the gap between these zones. This process converts the steady heat output from the person to a modulated sequence of positive and negative signals, spaced apart by gaps, which occur when the person lies between the detection zones. When this modulated signal exhibits the size and time characteristics that correspond with a person, an alarm signal is generated by the detector. As the detection zones for all of the lens segments are projected onto the same detectors, it is not possible to identify through which lens segment the energy is being focused, so the location of the object cannot be identified. When the person is moving within one of the detection zones or within one of the gaps, for example when moving towards the detector, no modulation is applied to the radiated energy and the movement of the person is not detected.
In higher performance detectors the array of lenses is often replaced with an array of mirrors, but as these are optically equivalent, the detection method is essentially the same.
In an array-based detector, the overall field of view can be determined in the same way as for a conventional camera, by placing the array on the focal plane of an appropriate lens. Consider a sensor using an array of 25 detectors arranged in a 5xc3x975 square. When the field of view is focused onto this array through a spherical lens, it is broken up into 25 xe2x80x9cpixelsxe2x80x9d in a square pattern, matching the array (see FIG. 2(b)). It is as if the overall field of view had been overlaid by a square grid, with each detector of the array viewing one square of the grid A1, A2 . . . B1, B2 etc. In contrast to conventional pyroelectric sensors, the field of view of each detector of an array (pixel) is contiguous with its neighbours, providing continuous coverage throughout the field of view.
The obvious method for detecting movement and position using an array is to detect the movement of an object (or the edge of an object) from the field of view of one detector to another. This restricts the resolution of the detection process to the size of the field of view subtended by each detector of the array. In the case of a 15xc3x9715 array placed at the focus of a spherical lens with a 90xc2x0 field of view, the field of view of each detector will subtend an arc approximately 1 m wide, at a distance of 10 m from the detector. As any movement of an object within this pixel is not detected, this sets a limit to the effective range that can be claimed when there is a requirement to detect a specified amount of movement by an object. If the detector were required to give an alarm with less than 0.5 m of movement by a person, the detector described above would have its effective range limited to less than 5 m. This issue is of importance in meeting regulatory requirements in certain applications areas.
The present invention can be used to determine the position and/or movement of an object within the field of view of a single detector in an array of detectors, thereby increasing the apparent resolution of the array. It also provides a mechanism for differentiating between static objects with modulated output energy, and objects oscillating about a mean position.
The proposed method applies to arrays constructed from single pieces of appropriate material and makes use of energy focused onto one detector of the array, being diffused onto adjacent detectors through the body of the material used to construct the array. This diffusion of energy has previously been considered a negative attribute of such detectors, as it reduces image sharpness. This invention turns this negative attribute into a benefit, expanding the capabilities of such detector arrays.
The present invention provides a method for determining the location and/or movement of an image within the field of view of one detector in an array of pyroelectric detectors constructed from a single piece of material and having an optical system for producing an image of an object on the array, comprising:
a) detecting the location of a first detector that contains a sub-pixel sized image,
b) selecting pairs of other detectors, adjacent to and diametrically opposed across the first detector;
c) for each selected pair of detectors comparing the magnitude of the signals from each of the the pair;
d) using the result of said comparisons to determine the position and/or movement of the image within the first detector.
There are many methods known in the prior art for locating an image that is no larger than one detector in an array of detectors, e.g. Vilaire et al in U.S. Pat. No. 5,229,594, so the method for achieving a) above will not be described here. In the following descriptions images are assumed to be of sub-pixel size unless otherwise stated.
In the preferred embodiment of the invention, the comparison (step (c)) comprises determining the ratio of the signal(s) from two detectors located opposite to each other on either side of the first detector. When the image is halfway between these detectors the ratio is equal, and when the image is closer to one detector the ratio moves to favour that detector and reduces in a corresponding manner for the other detector.
The method may also be used to determine the net movement of an object within the field of view of a first detector by comparing the signals of a pair of detectors, diametrically opposed across the first detector, by averaging the ratios of the signals over a period of time. An object which oscillates about a mean position in the field of view of the first detector will give rise to an equal ratio of the signals from the adjacent pair of detectors when their signals are averaged over a period significantly longer than the period of oscillation of the object.
The invention also provides a detector having means for carrying out the above methods.